The present invention relates to sensors for the detection of gas analytes and to sensor housing systems and, particularly, to electrochemical gas sensors and housing systems therefor.
The following information is provided to assist the reader to understand the invention disclosed below and the environment in which it will typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the present invention or the background of the present invention. The disclosure of all references cited herein are incorporated by reference.
Amperometric electrochemical gas sensors are electrochemical cells similar in structure and operation to batteries and fuel cells. As such, these three devices have several structures in common. In that regard, such devices include an anode, or anode compartment, where electrochemical oxidation occurs, a cathode, or cathode compartment, where electrochemical reduction occurs, an ionically conductive electrolyte, which maintains ionic electrical contact between the two electrodes, a housing, to enclose the electrodes and electrolyte, and contacts or poles, which are generally metallic electrical contacts between the electrodes and an external electronic circuitry used in connection with the devices. Batteries and fuel cells function primarily as power sources and place few design restrictions on the circuitry to which they may be in electrical contact. Amperometric gas sensors often require the use of specific driving circuitry, generally referred to as a potentiostat, for proper function. However, there are amperometric gas sensors that function analogously to fuel cells, and only require a method of measuring the electrical current which flows between the anode and cathode in the presence of the target analyte gas.
In an electrochemical gas sensor, the gas to be measured typically passes from the atmosphere into the sensor housing through a gas porous or gas permeable membrane to a first electrode known as a working electrode (sometimes called a sensing electrode) where a chemical reaction occurs. A complementary chemical reaction occurs at a second electrode known as a counter electrode (or an auxiliary electrode). The electrochemical sensor produces an analytical signal via the generation of a current arising directly from the oxidation or reduction of the analyte gas (that is, the gas to be detected) at the working and counter electrodes. A comprehensive discussion of electrochemical gas sensors is also provided in Cao, Z. and Stetter, J. R., “The Properties and Applications of Amperometric Gas Sensors,” Electroanalysis, 4(3), 253 (1992), the disclosure of which is incorporated herein by reference.
To be useful as an electrochemical sensor, a working and counter electrode combination must be capable of producing an electrical signal that is (1) related to the concentration of the analyte gas and (2) sufficiently strong to provide a signal-to-noise ratio suitable to distinguish between concentration levels of the analyte gas over the entire range of interest. In other words, the current flow between the working electrode and the counter electrode must be measurably proportional to the concentration of the analyte gas over the concentration range of interest.
In addition to a working electrode and a counter electrode, an electrochemical sensor often includes a third electrode, commonly referred to as a reference electrode. A reference electrode is used to maintain the working electrode at a known voltage or potential. The reference electrode should be physically and chemically stable in the electrolyte.
Electrical connection between the working electrode and the counter electrode is maintained through an electrolyte. Important functions of the electrolyte include: (1) to efficiently carry the ionic current; (2) to solubilize the analyte gas; (3) to support both the counter and the working electrode reactions; and (4) to form a stable reference potential with the reference electrode. Important criteria for an electrolyte include the following: (1) electrochemical inertness; (2) ionic conductivity; (3) chemical inertness; (4) temperature stability; (5) low cost; (6) low toxicity; (7) low flammability; and (8) appropriate viscosity.
In general, the electrodes of an electrochemical cell provide a surface at which an oxidation or a reduction reaction occurs to provide a mechanism whereby the ionic conduction of the electrolyte solution is coupled with the electron conduction of the electrode to provide a complete circuit for a current.
The measurable current arising from the cell reactions of the electrochemical cell is directly proportional to the extent of reaction occurring at the electrode. Preferably, therefore, a high reaction rate is maintained in the electrochemical cell. For this reason, the counter electrode and/or the working electrode of the electrochemical cell generally comprise an appropriate electrocatalyst on the surface thereof to support the reaction rate.
Batteries are completely self-contained electrochemical energy storage and conversion devices. They are arranged so that both the anode and the cathode are comprised of or are in intimate electrical contact with relatively large quantities of substances with sufficiently different electrochemical energies such that when the anode and cathode poles are connected to electronic circuitry, significant and useful amounts of electrical current flow through the circuitry. The source of this current is the electrochemical conversions of the anode and cathode materials (oxidation and reduction). Thus, batteries are a useful electrochemical device. Batteries are entirely self-contained from an electrochemical point of view in that they are fabricated with sufficient anode and cathode material to provide a useful lifetime or amount of electrical energy. As such, batteries are usually well sealed. In many designs, they are hermetically sealed. Common examples of batteries include the Leclanche' cell (the ‘dry’ cell) and the Plante' cell (the lead acid battery).
Fuel cells, on the other hand, are electrochemical energy conversion devices that require an external supply of the anode material, the cathode material or both. The electrodes of fuel cells are usually electrocatalytic in nature (that is, they provide electrochemically active surfaces to support the electrochemical reactions, but do not actually chemically participate in them). Unlike batteries, for which the useful life generally ends when the electrochemically active electrode materials are consumed, a fuel cell will operate continuously as long as electrochemically active fuel (anode material) and oxidizer (cathode material) are supplied to the device. A common fuel cell is the Grove cell, or hydrogen-oxygen fuel cell. In that fuel cell, hydrogen is the fuel and oxygen is the oxidizer.
Amperometric electrochemical gas sensors can be considered special cases of fuel cells in that they are typically miniature cells (compared to power generation fuel cells) that are designed to use a target gas or analyte gas (that is, a gas of analytical interest) as the fuel. In the absence of the target gas, there are no bulk electrochemical conversions (Faradaic reactions) occurring at the electrodes of the sensor and, hence, essentially zero current flows in the sensor. When present, the target gas undergoes electrochemical oxidation or reduction as described above, with corresponding generation of Faradaic currents. The resultant current flow is sensed by the external driving circuitry and is the analytical signal of the sensor. Once again, the observed current is typically directly proportional to the concentration of the analyte gas present.
Although batteries, fuel cells and amperometric electrochemical gas sensors are very similar, the manufacture of amperometric gas sensors poses several unique difficulties. First, unlike batteries, there must be a gas entry to allow the ingress of the analyte gas. There must also be contacts or poles which carry the current from the surfaces of the electrodes to the external circuitry. Finally, the sensor must be fabricated in such a way as to retain the ionic electrolyte, often a highly corrosive aqueous acid or base. Methods and systems for sealing such sensors against leakage of the internal liquid electrolyte while allowing entry of the analyte gas and collection of the resultant currents are an important feature of the mechanical design of such sensors.
Sealing methods in currently available amperometric electrochemical gas sensors include compressible o-rings, adhesives, sealants, even battery-type housings, either individually or in combination. Generally, the electrochemical gas sensors also include an electronic current path to carry the analytical currents generated at the electrodes to the external circuitry. Such electronic current paths typically take the form of metallic pins, wires or ribbons that penetrate the sensor housing to carry the current from the electrodes. Such metallic element provide a pathway for leakage over the lifetime of the sensor.
Electrically conductive plastics have been used to form all or part of a sensor housing in an attempt to provide an efficient electrical current path from the electrode surfaces to the external circuitry without creating a potential leak path from the interior of the sensor housing. Such conductive plastics are generally homogeneous with respect to conductivity in that they exhibit conductivity throughout an article, component or part formed from the conductive plastic. Such plastics enable conduction of electricity from the inside of a sensor housing to the outside. Conductive plastics portions have, for example, been insert injection molded into typical plastic sensor housings and have been incorporated into the sensor housing by adhesives, welding, heating, etc. The conductive portions of the sensor housing serve both to carry current from the electrodes of the sensor to the external circuitry and to form part of the structure and sealing system of the sensor. The conductive polymers or plastics used in sensors can be relatively expensive and difficult to use in manufacturing procedures.
Although there have been a number of methods and systems developed to form electronic current paths from electrodes within an electrochemical sensor, it is desirable to develop improved methods and systems of providing such electronic current paths.